Mucocutaneous lymph node syndrome (MCLS) is an acute febrile illness of unknown etiology associated with mucosal inflammation, polymorphous skin rash, and coronary artery aneurysms. Recently, we studied the immunologic status of peripheral blood mononuclear cells from these patients and found that the acute phase of MCLS was characterized by a deficiency of circulating suppressor/cytotoxic T cells, presence of activated helper T cells, increased spontaneous secretion of IgG and IgM and cytotoxicity against normal skin fibroblasts. With few exceptions, each of these 3 abnormalities was markedly improved when patients with MCLS were studied during convalescence. We propose to study the immunoregulatory abnormalities in acute MCLS and to relate them to the pathogenesis of the disease. Specifically, to determine the relationship of the immunoregulatory T cell imbalance to B cell activation and to fibroblast cytotoxicity; To grow activated T cells from patients with acute MCLS in continuous culture with T cell growth factor and study their specificity for antigens suspected of precipitating the disease and their effect on normal B cells and on the activity of cytotoxic cells; to characterize the fibroblast cytotoxic cell and to develop a technique for assaying cytotoxic cells or antibodies directed against arterial cells; and to study the phenotypic characteristics of the cellular infiltrate in the skin lesion. It is hoped that these studies will contribute to a better understanding of the cause and pathogenesis of MCLS, provide a basis for more objective criteria in the diagnosis of this disease, and result in more specific treatment of those patients with a prolonged complicated course of MCLS.